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(CNN)The bodies of two UN experts who went missing in the Democratic Republic of Congo this month have been found, the United Nations said in a statement Tuesday.

UN Secretary-General Antonio Guterres said UN peacekeepers discovered the bodies of US citizen Michael Sharp and Swedish national Zaida Catalan on Monday outside the city of Kananga. The body of their Congolese interpreter, Betu Tshintela, was also discovered with them.

A Congo government official told CNN that Catalan's body was found decapitated, but Sharp and Tshintela were not beheaded. Their cause of death is not determined, according to government spokesman Lambert Mende.

"They were found together with the body of a Congolese interpreter who was working with them and who was identified clearly by our police," Mende said.

The bodies are at a municipal hospital in Kasai province and Mende said their remains will likely be transferred to Kinshasa on Thursday.

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Sharp and Catalan were members of the UN Group of Experts on Congo who were investigating large-scale human rights violations in the region.

Ben Wideman, a Mennonite pastor, paid tribute to his college friend Sharp. "Even back in college it was clear that MJ would be a person who would head out to save the world," he wrote, using Sharp's nickname. "May we all strive to live out our lives like MJ, working for peace wherever we are called."

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In a statement, Guterres said, "Michael and Zaida lost their lives seeking to understand the causes of conflict and insecurity in the DRC in order to help bring peace to the country and its people."

The UN will be launching an investigation into the cause of their deaths, the statement added, and urged the DRC government to conduct its own investigation. Efforts to find the four Congolese nationals who also went missing should continue, the statement said.

The US' ambassador to the UN, Nikki Haley, released a statement offering condolences to the two UN workers' families.

"It is always difficult to lose a brave American dedicated to service," Haley's statement reads. "Michael was working on the front lines of what we try to do at the United Nations every day: find problems and fix them."

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Ida Sawyer, Central Africa director at Human Rights Watch said the team's disappearance reflected a bigger picture of the violence in the Kasai region of the DRC.

"The Human Rights Council should establish a commission of inquiry into abuses in the region as soon as possible. Concerted efforts are urgently needed to address this increasingly desperate situation," she said.